Presently, several methods exist for reducing nicotinic alkaloids, such as nicotine, in plants. A low-nicotine strain of tobacco has been employed, for instance, as breeding stock for low-nicotine cultivars. Legg et al., Crop Sci 10:212 (1970). Genetic engineering methods also can be used to reduce nicotine levels. For example, U.S. Pat. Nos. 5,369,023, and 5,260,205 discuss decreasing nicotine levels via antisense targeting of an endogenous putrescine methyl transferase (PMT) sequence. Voelckel et al., Chemoecology 11:121-126 (2001). The tobacco quinolate phosphoribosyl transferase (QPT) gene has been cloned, Sinclair et al., Plant Mol. Biol. 44: 603-617 (2000), and its antisense suppression provided significant nicotine reductions in transgenic tobacco plants. Xie et al., Recent Advances in Tobacco Science 30: 17-37 (2004). See also U.S. Pat. Nos. 6,586,661 and 6,423,520.
Several nicotine biosynthesis enzymes are known. For instance, see Hashimoto et al., Plant Mol. Biol. 37; 25-37 (1998); Reichers & Timko, Plant Mol. Biol. 41:387-401 (1999); Imanishi et al., Plant Mol. Biol. 38:1101-1111 (1998). Still, there is a continuing need for additional genetic engineering methods for further reducing nicotinic alkaloids. When only PMT is down-regulated in tobacco, for example, nicotine is reduced but anatabine increases by about 2-to-6-fold. Chintapakorn & Hamill, Plant Mol. Biol. 53: 87-105 (2003); Steppuhn, et al., PLoS Biol 2(8): e217: 1074-1080 (2004). When only QPT is down-regulated, a fair amount of alkaloids remain. See U.S. Plant Variety Certificate No. 200100039.
Reducing total alkaloid content in tobacco would increase the value of tobacco as a biomass resource. When grown under conditions that maximize biomass, such as high density and multiple cuttings, tobacco can yield more than 8 tons dry weight per acre, which is comparable with other crops used for biomass. Large-scale growing and processing of conventional tobacco biomass has several drawbacks, however. For example, significant time and energy is spent extracting, isolating, and disposing tobacco alkaloids because conventional tobacco biomass, depending on the variety, contains about 1 to about 5 percent alkaloids. On a per acre basis, conventional tobacco biomass contains approximately as much as 800 pounds of alkaloids. Also, people handling tobacco may suffer from overexposure to nicotine, commonly referred to as “green tobacco disease.”
Reduced-alkaloid tobacco is more amenable for non-traditional purposes, such as biomass and derived products. For example, it is advantageous to use reduced-alkaloid tobacco for producing ethanol and protein co-products. U.S. published application No. 2002/0197688. Additionally, alkaloid-free tobacco or fractions thereof may be used as a forage crop, animal feed, or a human nutritive source. Id.
Beyond these benefits associated with reducing nicotine, more successful methods are needed to assist smokers in quitting smoking. Nicotine replacement therapy (NRT) is not very effective as a smoking cessation treatment because its success rate is less than 20 percent after 6 to 12-months from the end of the nicotine replacement period. Bohadana et al., Arch Intern. Med. 160:3128-3134 (2000); Croghan et al., Nicotine Tobacco Res. 5:181-187 (2003); Stapleton et al., Addiction 90:31-42 (1995). Nicotine-reduced or nicotine-free tobacco cigarettes have assisted smokers in quitting smoking successfully, by weaning the smoker from nicotine yet allowing the smoker to perform the smoking ritual. Additionally, denicotinized cigarettes relieve craving and other smoking withdrawal symptoms. See Rose, Psychopharmacology 184: 274-285 (2006) and Rose et al., Nicotine Tobacco Res. 8:89-101 (2006).
Accordingly, there is a continuing need to identify additional genes whose expression can be affected to decrease nicotinic alkaloid content.